Shallow Shear‐Wave Reflection Surveys in the Wellington Central Business District

Seismic reflection profiling using shear waves provides constraints on the thickness of sediments beneath the Wellington Central Business District. Standard seismic methods were modified for urban environments where limited grassed areas are available and levels of cultural noise are high. Two‐way travel times on the stacked seismic section were converted to depth using a site‐specific velocity model based on direct velocity measurements from local downhole seismic surveys. Basement depths at Wellington Girls’ College and Waitangi Park are estimated to be 180 ± 19 and 166 ± 16 m, respectively. At Wellington Girls’ College, near‐surface sediments are horizontally layered, whereas at Waitangi Park, sediment layers are tilted in a zone of distributed, steep faulting. This fault zone is interpreted to be the onshore extension of the Aotea Fault that has been identified in Wellington Harbour on marine seismic reflection profiles. At Miramar Polo Ground, collocating a seismic survey with a logged borehole enabled us to benchmark our seismic interpretations against subsurface geology. Our new estimates of basement depth in the Wellington basin provide vital depth constraints for models that predict how shaking from earthquakes can vary across the city due to the basin's geometry.